1. Field of the Invention
The invention relates to an electromagnetic deflection unit for a cathode ray tube, comprising:
a hollow, annular support having a constricted and a wide end and a longitudinal axis;
a flange at the constricted and the wide end, respectively, of the support, each flange having at least one tangential groove with a bottom and each having a multitude of substantially radial grooves merging into a said tangential groove;
a first set of deflection coils for line deflection of an electron beam in a first direction at right angles to the longitudinal axis, which deflection coils are directly wound on the inside of the support and whose turns each run through the tangential groove and through radial grooves in the flanges; and
a second set of deflection coils for field deflection of an electron beam in a direction at right angles to the longitudinal axis and at right angles to the first direction, which deflection coils are directly wound on the support and whose turns run through radial grooves in the flanges.
2. Description of the Related Art
A deflection unit of this type is known from European Pat. No. 0,102,658A1 (PHN 10416) which corresponds to U.S. Pat. No. 4,484,166, issued Nov. 20, 1984.
Cathode ray tubes have a neck-shaped portion one end of which accommodates an electron gun and the other end of which merges into a tapered portion with a screen contiguous to it. An electromagnetic deflection unit surrounds the neck-shaped portion and rests against the tapered portion or is positioned at a short distance therefrom. In the case of a colour display tube this deflection unit must be capable of deflecting the electron beams to the corners of the screen while maintaining convergence. This means that both the horizontal deflection field and the vertical deflection field must have a very special distribution. To realize this, the known deflection unit is provided between its ends with an annular body having guide grooves in the inner circumference accommodating the longitudinal segments of the coil turns. This provides a possibility of controlling the wire distribution (and hence the field distribution): the choice is not restricted to wires running straight from front to back but they may alternatively run in a bend via the grooves in the intermediate ring. The wire location of a coil can therefore be freely modulated as a function of the direction along the longitudinal axis in the direction of the corners and a self-converging deflection coil system can be realized.
Since both the wires of the line deflection coil and the field deflection coil are guided on the inside of the intermediate ring and are thus positioned close together, there is a risk of ringing occurring between the line deflection coil and the field deflection coil.
Since a limited number of grooves can be provided in the inner circumference of the said ring, there may be a number of grooves, dependent on the coil design, accommodating longitudinal turn segments of both the line deflection coil and of the field deflection coil. During winding, for example, the field deflection coil turns are accommodated in these grooves first and then the line deflection coil turns are accommodated. In addition to the risk of ringing there is also the risk of high voltage breakdown between the line and field deflection coils.